This invention relates to an improvement of a liquid encapsulated Czockralski method (LEC method) for growing a single crystal of compound semiconductors.
In growing a single crystal of compound semiconductors, various kinds of elements are usually doped in order to change electronic characteristics or to reduce the dislocation density of the single crystal.
For example some elements which have a different valence number from that of matrix elements of the semiconductor crystal are doped as impurity in order to make the grown crystal into an n-type semiconductor or a p-type semiconductor.
In this invention compound semiconductors mean semiconductors of groups III-V and groups II-VI in the periodic table.
For example GaAs, GaP, InP, GaSb, etc are semiconductors of groups III-V. CdS, CdSe, CdTe, etc are semiconductors of groups II-VI.
For example in the case of growing a single crystal of GaAs, isoelectronic impurities--B, In, Sb, etc--are doped in order to reduce the dislocation density in the single crystal. Isoelectornic impurity is defined as an impurity whose valence number is the same with one of the matrix elements of semiconductor crystals.
Besides, S, Te, Si, etc which are not isoelectornic impurities are doped into GaAs single crystals to change electronic characteristics.
A liquid encapsulated Czockralski method (LEC method) is one of the prevailing methods to grow a single semiconductor crystal.
An LEC method comprises; melting encapsulant material and compound material into a compound melt covered with a liquid encapsulant in a crucible by heating, dipping a seed crystal into the compound melt, growing a single crystal from the compound melt by pulling up and rotating the seed crystal and cooling the grown crystal in a cooling zone above the crucible.
In the case of doping some impurities, the impurities are added into the compound material as elements or compounds which comprise the impurity elements.
When a single crystal is grown from a compound melt which includes an impurity element, the impurity concentration C.sub.s in a solidified single crystal pulled up is not equal to the impurity concentration C.sub.L in the compound melt in general.
The boundary between a compound melt and a solidified single crystal is called a liquid-solid interface. Generally the ratio of the impurity concentrations of a solidified part to that of a melt is a constant value, which is called a distribution coefficient.
The distribution coefficient depends upon a pressure acting on the melt and a ratio of elements of matrix compound in the melt. But if the pressure is kept constant, the distribution coefficient is constant in the crystal growth.
The distribution coefficient k is defined by ##EQU2## If the impurity concentration is 1 in a melt, the impurity concentration of the solidified part at the liquid-solid interface is k.
Distribution coefficients are defined by determining an impurity element and a matrix melt. They obtain various values according to the impurity element and the matrix melt. When the impurity element and the matrix melt are identified, the distribution coefficient changes as a function of pressure.
But in many cases the distribution coefficient is smaller than 1. If the impurity has a distribution coefficient smaller than 1, impurity atoms do not easily penetrate into the solidified part. When a single crystal is pulled up from a compound melt including an impurity element by an LEC method, the compound elements of the matrix of the crystal are deprived from the crucible more rapidly than the impurity element. Then the impurity concentration in the melt gradually increases during the crystal growth.
The impurity concentration C in a crystal grown by an LEC method is given by EQU C=C.sub.0 k(1-g).sup.k-1 ( 2)
Where C.sub.0 is an initial impurity concentration in the compound melt and g is a ratio of the solidified part to the initial compound melt by weight. This ratio is called the solidification ratio henceforth for simplicity.
At the initial state the solidification ratio g is zero. During the crystal growth the solidification rate g increases.
If the distribution coefficient k is smaller than 1, the impurity concentration C is lowest at the beginning of the crystal growth because the solidification rate g is zero. And the impurity concentration C raises as the crystal growth proceeds. When the solidification rate g comes close to 1, the impurity concentration diverges.
Accordingly if the impurity having a distribution coefficient k less than 1 is doped in a compound melt, the impurity concentration C is lowest at a seed end of a single crystal grown from the melt and is highest at a tail end of the crystal.
A single crystal ingot grown by the LEC method is sliced in the planes which are perpendicular to the growth axis. Sliced crystals are called wafers. According to the explanation abovementioned, the impurity concentration is different with regard to each wafer sliced from the same crystal ingot. Therefore it is difficult to make many wafers having the same characteristics by the LEC method.
Furthermore when the impurity concentration in the initial melt is very high, the impurity concentration in the melt raises higher than a limit of single-crystallization. Separation of impurity atoms on the surface of the pulled crystal happens. After the separation occurs, the solidified part does not become a single crystal. Therefore only a small upper portion of the ingot is available, because a semiconductor wafer must be a single crystal.
It is desirable that the grown ingot should be a single crystal from seed end to tail end and the impurity concentration should be uniform in the single crystal.
If a great amount of compound melt which is many times larger than the crystal to be grown are contained in a big crucible, the change of the impurity concentration during the crystal growth might be trivial.
But in practical cases the diameter of a crucible is determined to be twice as big as the diameter of the single crystal grown from the crucible, and the depth of the crucible is nearly equal to the diameter of the crystal. Thus it is impossible to use an excessive amount of compound melt.